The invention relates to a cathode material for fluoride-based conversion electrodes, a method for producing said material and its use.
Future mobile and portable device applications require secondary batteries with high energy densities, wherein batteries operating on the basis of the lithium-ion exchange are particularly suitable for this. J.-M. Tarascon and M. Armand, Nature 414, p. 359, 2001; S.-Y. Chung et al., Nature Materials 1, p. 123, 2002, K. Kang et al., Science 311, p. 977, 2006, as well as M. Armand and J.-M Tarascon, Nature 451, p. 652, 2008 conducted experiments in this field and developed materials which can reversibly store lithium.
Batteries of this type contain cathode materials with layered structures or, optionally, provide the option of storing lithium, especially LiCoO2, LiMnO2 and LiFePO4. With working voltages of 3.3-4.0 V, they have for the most part specific capacities of 90-140 mAh/g. Intercalation materials such as LiNiO2 and LiMn2O4 are of interest because they are cheaper and have a lower environmental impact. However, higher storage densities are not reported. On the anode side, graphite and related carbon materials are primarily used as highly reversible intercalation systems for lithium, thus making it possible to achieve specific capacities of up to 373 mAh/g.
Given this background, it is the goal of the present invention to develop new batteries which can be produced cost-effectively, have a low impact on the environment, can be handled safely and exhibit little sensitivity to temperature influences, have a lasting element composition, permit a high cycle number and have a high gravimetric (Wh/kg) as well as a high volumetric energy density (Wh/l).
It was recently discovered that especially high energy densities can be achieved when using electrode materials which operate on the basis of conversion materials instead of storage materials. On the anode side, Y. Oumellal et al., Nature Materials 7, p. 916, 2008 and P. G. Bruce et al., Angew. Chem. Int. Ed. [Applied Chemistry, Int. Ed.] 47, p. 2930, 2008, have already achieved first successes with oxides such as SnO2 or MoO3 and metal hydrides. In those cases, it was demonstrated that the active material is reduced to the metal, wherein a lithium compound is formed parallel thereto. During the dilithiation, the direction of the reaction is reversed and the metal is again oxidized.
On the cathode side, conversion materials on the basis of metal fluorides offer high theoretical potentials which can reach up to 2000 mAh/g, wherein the following reversible electro-chemical reaction in principle takes place in the process:nLi++ne−+Men+FnnLiF+Me°
Metallic lithium consequently reacts during the discharging operation with the fluoride of a transition metal Me, thereby forming lithium fluoride and the transition metal. The anode consists of lithium, e.g. in metallic form or intercalated into graphite, while the cathode consists of metal fluoride and a conductive carbon material.
H. Arai, Sh. Okada, Y. Sakurai, J. Yamaki in J. Power Sources, 68, p. 716, 1997, for the first time report on the high theoretical potential of metal trifluorides. However, at room temperature and using an electrode material composed of FeF3 and acetylene soot, they achieved only a specific capacity of 80 mAh/g.
Higher capacities were described in the document US 2004/0121235 A1 and by F. Badway et al. in J. Electrochem. Soc. 150, p. A1209, 2003, and in J. Electrochem. Soc. 150, p. A1318, 2003 for a carbon-metal composite, for example composed of 85% by weight FeF3 and 15% by weight C, which allowed achieving a specific capacity of 200 mAh/g at room temperature, corresponding to the reversible reaction of Fe3+ to Fe2+ in the range of 2.8-3.5V. The improved properties as compared to Arai et al. were attributed to the fact that the metal fluoride in said case was present in a smaller grain size and that it was thoroughly mixed with a conductive carbon material. Different amounts of graphite, soot and active carbon were mixed with the metal fluoride and the resulting mixture was ground for several hours in a high-energy ball grinder. The resulting crystallite size for the FeF3 was listed as 30-50 nm. Specific capacities of up to 560 mAh/g could be achieved in this way. However, the ability of the material to cyclize was limited and the measurements had to be taken at 70° C. because of the poor kinetics of the electrode processes at room temperature. The document US 2004/0121235 A1 furthermore discloses that nanocomposites composed of lithium fluoride, a transition metal and elementary carbon can be used as reversible electrode material.
Bervas et al., Electrochem. Solid State Lett. 8, p. A179, 2005 report on a reversible reaction of a BiF3/C nanocomposite by forming Bi and LiF at a specific capacity of 230 mAh/g. However, this material also does not exhibit good cycle properties.
With the example of TiF3 and VF3, H. Li, G. Richter and J. Maier, Advanced Materials 15, p. 736, 2003, demonstrated that transition metals which form alloys with lithium can be cyclized better and allow higher cycle numbers than other metals such as iron which do not form alloys and quickly lose capacity. A specific capacity of up to 500 mAh/g after 10 cycles was reported for TiF3 and VF3 cathodes, wherein the electrode material was produced with the aid of ball grinders and using the starting materials.
In App. Surf. Sci. 252, p. 4587, 2006 and in Electrochem. Commun. 8, 1769, 2006, Makimura et al. report on the production of a FeF3 film which is thinner than 1 μm and was deposited via pulsed laser desorption onto a substrate cooled to −50° C. Additional films, deposited at 600° C., were composed of FeF2. The two films differed in their initial electro-chemical behavior but adapted after several cycles to the behavior of the FeF2.
The methods used so far to produce cathode materials used in fluoride-based batteries required that the constituents were either mechanically alloyed with the aid of high-energy ball grinders or were deposited with the aid of thin-film processes. These methods obviously result in limited cyclization, restrictions in the material selection, and a lowering of the measured specific capacity. The use of metal fluoride cathodes is therefore made more difficult as a result of partial irreversibility of the electrode processes and poor cycle properties.
Leonhardt et al. disclose in Chemical Vapor Deposition 12, p. 380, 2006, that gaseous ferrocene subjected to a carrier gas process will have decomposed completely at temperatures above 500° C., in accordance with the following reaction:Fe(C5H5)2→Fe+H2+CH4+C5H6+ . . . +reactive hydrocarbons.
Depending on the flow rate for the carrier gas, the iron clusters behave in the manner of catalytically functioning nuclei for producing different types of nanocarbons, also including single-wall or multi-wall carbon nanotubes and hollow carbon fibers, which grow on the catalyst nuclei.
Additional techniques for producing nanocarbon materials include the arc discharge (see Ebessen et al., Nature 358, p. 220, 1992), the laser ablation (see Thess et al., Science 273, p. 483, 1996) and the chemical vapor deposition (CVD; see Jose-Yacaman et al., Appl. Phys. Lett. 62, p. 657, 1993) as well as the so-called HiPco process (Nikolaev et al., Chem. Phys. Lett. 313, p. 91, 1999).
A method for producing carbon fibers is known from the document U.S. Pat. No. 6,946,110 B2, wherein benzene is decomposed in an inert gas flow at 1200° C., in the presence of an organic compound and a transition metal catalyst.
Hu et al., Adv. Func. Mater. 17, p. 1873, 2007, demonstrated that using a micro-porous and nano-porous carbon material offers advantages for the electro-chemical applications. The carbon material described therein was produced with the aid of an involved template process and using a SiO2 matrix.
According to the document U.S. Pat. No. 6,465,132 B1, carbon nanofibers and nanotubes are produced from ferrocenes with the aid of reactions taking place in a gas mixture which is conducted with a carrier gas through a typical CVD [chemical vapor deposition] reactor of quartz glass. It is critical for this operation that a plurality of reaction parameters are adhered to precisely, in particular the temperature, the reaction time, the concentration of the precursor, and the flow rate of a carrier gas.